1. Field of the Invention
The present invention relates to a television tuner, and more particularly to a television tuner suitable for use in television receivers for receiving analog television broadcasts or digital television broadcasts.
2. Description of the Prior Art
A television tuner according to the prior art will be described with reference to FIG. 5. Analog television signals and digital television signals are inputted to an input tuning circuit 31. The tuning frequency of the input tuning circuit 31 is so controlled as to be tuned to television signals on the channel to be received, but the tuning characteristic is relatively loose because it is configured of a single-tuned circuit. At the stage immediately following the input tuning circuit 31 is connected a high frequency amplifier 32. The high frequency amplifier 32 is made up of a variable gain amplifier, whose gain can be varied with an AGC voltage that is applied. At the stage immediately following the high frequency amplifier 32 is connected an inter-stage tuning circuit 33. Although the inter-stage tuning circuit 33, too, is so controlled as to be tuned to television signals on the channel to be received, the tuning characteristic is sharper than that of the input tuning circuit 31 because it is configured of a double-tuned circuit. At the stage immediately following the inter-stage tuning circuit 33 is connected a mixer 34. The mixer 34 is supplied with local oscillation signals from an oscillator 35.
In the configuration described above, whereas the inputted television signals undergo frequency conversion into intermediate frequency signals by the mixer 34, only the television signals of the channel to be received are converted into a frequency range in a prescribed intermediate frequency band. Whereas other television signals than those of the channel to be received are converted outside the range in the prescribed intermediate frequency band, the level of intermediate frequency signals deriving from them is lower than that of intermediate frequency signals in the range because they attenuate according to the tuning characteristics of the input tuning circuit 31 and the inter-stage tuning circuit 33.
At the stage immediately following the mixer 34 is connected a first intermediate frequency amplifier 36, and at the stage immediately following the first intermediate frequency amplifier 36 is connected a SAW filter 37. At the stage immediately following the SAW filter 37 is connected a second intermediate frequency amplifier 38.
At the stage immediately following the SAW filter 37 is also connected detecting means 39, and at the stage immediately following it is connected an operational amplifier 40. The output end of the operational amplifier 40 is connected to the gain control end of the high frequency amplifier 32.
In the foregoing configuration, the SAW filter 37 has a sharp characteristic to pass almost only the intermediate frequency band and to attenuate all other ranges to a considerably low level. Therefore, out of the intermediate frequency signals outputted from the SAW filter 37, the level of intermediate frequency signals outside the pertinent range in the intermediate frequency band is far lower than that of intermediate frequency signals within that range. The detecting means 39 detects intermediate frequency signals in the range and outputs them as an AGC voltage. The AGC voltage, after being converted to a prescribed level by the operational amplifier 40, is supplied to the high frequency amplifier 32. Therefore, intermediate frequency signals within the range corresponding to the television signals of the channel to be received are outputted at a substantially constant level from the second intermediate frequency amplifier 38. The outputted intermediate frequency signals are demodulated by a demodulator (not shown) for analog or digital signals provided at a subsequent stage.
Incidentally, whereas digital television signals are transmitted over some other channel than the channel for conventional analog television signals, their level may become 35 dB to 45 dB lower than that of the analog television signals as shown in FIG. 6. When signals transmitted over a channel for digital television signals in such a state are received, even if analog television signals of a great field strength are present on a nearby channel, the level of intermediate frequency signals outside the pertinent range based on the analog television signals outputted from the SAW filter will be lower than that of intermediate frequency signals within the range based on the digital television signals. Therefore, no AGC operation would take place on the basis of the intermediate frequency signals outside the range.
Accordingly, at a stage where the level of television signals of the channel to be received requires no AGC operation, analog television signals amplified to the maximum by the high frequency amplifier are inputted to the mixer to generate a heavy distortion in the mixer. There may arise a further problem that, if the high frequency amplifier is saturated by being oscillated in a large amplitude, gain compression will occur to make it impossible to sufficiently amplify the digital television signals to be received.
This problem can be solved by so setting as to start the AGC operation at a stage where the level of the television signals to be inputted is still low, but there would arise a new problem that the S/N ratio of intermediate frequency signals of the channel to be received deteriorates.